1. Field of The Invention
The present invention is directed to an optical print head of the type that utilizes optical fibers and a light source to record images and text onto a recording medium, and more particularly to an improved optical print head which utilizes optical fibers coupled to closely positioned optical waveguides for improving the speed of recording.
2. State of The Prior Art
One method of increasing the writing speed of an optical printing system is to increase the number of channels used to transfer information onto the recording medium and to have them write in parallel. If for some reason the channels cannot be separated by the desired distance, the print head containing the channels can be tilted at an angle and the electronic signals modulating the light to the channels delayed by the amount needed to compensate for the tilt. Large tilt angles are needed if the spacing between the channels is too large and the accurate setting of the print head then becomes difficult.
A patent of interest for its teaching is U.S. Pat. No. 4,389,655, entitled "Optical Device For Non-Contact Recording And Particular Facsimile Reproduction Of Images And Test" by P. Baues. In that patent, optical fibers are held in close proximity by the use of grooves in a supporting structure. Another approach for decreasing the spacing between each printing element is to form the printing elements from a fanned out plurality of closely spaced optical waveguides. The fanned out end of each of the waveguides is connected to an individual optical fiber. The opposite ends of the waveguides are closely positioned, at distances which are smaller than the diameters of the optical fibers. Such a structure is suggested in FIG. 4 of a paper by J. T. Boyd and S. Sriram entitled "Optical Coupling from Fibers to Channel Waveguides formed on Silicon" Mar. 15, 1978/ Vol. 17, No. 6/Applied Optics.
A number of problems occur when channel waveguides are used in a print head, particularly when optical waveguides are formed in substrate mediums such as glass, plastic or LiNbO.sub.3 with each optical waveguide positioned in close proximity to its neighbor. One of the problems is that the Gaussian distribution of the intensity of the light from the end of the waveguide is skewed in a direction perpendicular to the surface of the waveguide. This is caused by the presence of the material forming the optical waveguides such as Ti which increases the index of refraction at its location. The index of refraction then decreases monotonically from a maximum at the surface to a bulk value (associated with the particular medium used) at a distance of several microns below the surface. This causes the optical beam, formed on the recording medium in this direction, to take on a skewed gaussian shape when in fact a symmetric gaussian shape is preferred. A second problem is cross talk which occurs because the elements forming the optical waveguide are close to each other.
The first problem, namely the one caused by the difference in the index of refraction of the materials used to form the print head may be minimized when using LiNbO.sub.3 by diffusing a thin layer of MgO over the Ti. A paper which addresses a solution to this problem is authored by J. Noda et al and is entitled "Effect of Mg Diffusion on Ti-diffused LiNbO.sub.3 Waveguides", J. Appl. Phys., Vol. 49, No. 6, June 1978.
A suggestion of a solution to the second problem is set out in a paper authored by J. L. Jackel et al, and entitled "Nonsymmetric Mach-Zehnder Interferometers Used as Low-Drive-Voltage Modulators". In that paper it is stated that optical decoupling of the waveguides may be effected by differing the propagation constants of the two guides. One method for accomplishing this difference is to provide different widths for each waveguide.
In order to provide an optical head having multiple waveguide channels it is necessary to address and to solve the aforementioned problems.